1. Field of the Invention
The present invention is directed toward the field of transmission line filters.
2. Description of the Related Art
In data communications systems, data is transferred over transmission lines at high frequencies. For example, in a data communications network that complies with the Institute of Electrical and Electronics Engineers ("IEEE") 802.3u Standard for data communications, differential three level analog baseband signals are transferred over transmission lines at a rate of 125 megahertz ("MHZ").
The IEEE 802.3u Standard supports both a 100 Base-T4 standard and a 100 Base-TX standard. In 100 Base-T4, Category 3 type twisted pair wire having lengths up to 100 meters is used to transmit data. In 100 Base-TX, either Category 5 shielded or Category 5 unshielded twisted pair wire having lengths up to 100 meters is used to transmit data.
During a high frequency transmission of signals, such as the signal transmissions in IEEE 802.3u 100 Base-T4 and 100 Base-TX networks, signals become severely attenuated and undergo significant phase shifts. FIG. 1 shows two graphs 100 and 110. Graph 100 illustrates the loss of amplitude that high frequency signals suffer on different lengths of the Category 5 unshielded twisted pair cable. The vertical axis in graph 100 measures signal amplitude loss in decibels ("db"), and the horizontal axis measures the frequency of the signal on the transmission line.
Curve 101 shows the characteristics of a 100 meter Category 5 unshielded twisted pair cable. For signals in the range of 100 MHZ, the amplitude loss is severe at -20 db. Curves 102, 103, and 104 in graph 100 illustrate the characteristics of Category 5 unshielded twisted pair cables having lengths of 50 meters, 25 meters, and 1 meter, respectively. In the case of curves 102 and 103, the amplitude loss is also shown to be fairly significant at frequencies in the range of 100 MHZ.
Graph 110 illustrates the phase shift that signals undergo at different frequencies on Category 5 unshielded twisted pair cable. Curves 111, 112, 113, and 114 show the phase shift characteristics of Category 5 unshielded twisted pair cables having lengths of 100 meters, 50 meters, 25 meters, and 1 meter, respectively. At a frequency in the range of 100 MHZ, the phase shift for the 100 meter cable 111 exceeds 100 degrees. Similarly undesirable phase shifts are shown in curves 112 and 113.
FIG. 2 illustrates the distortion that is suffered by a differential three level analog signal in a 100 Base-TX IEEE 802.3u compliant network. FIG. 2 shows two streams of bits 120 and 121 each being transmitted on an IEEE 802.3u Standard 100 Base-TX Category 5 unshielded twisted pair cable at a frequency of 125 MHZ. A measure of time is provided on a horizontal axis below the signals in each bit stream 120 and 121.
Bit stream 120 is a set of bits represented by differential three level analog signals afer traveling a distance of 1 meter on a 100 Base-TX Category 5 unshielded twisted pair cable. Bit stream 121 shows the same bits from bit stream 120 after traveling a distance of 100 meters on the same cable. As can be seen from FIG. 2, signals being transferred at 125 MHZ over 100 meters of 100 Base-TX Category 5 unshielded twisted pair cable become very distorted due to both amplitude attenuation and phase shift.
In order to properly receive signals that are transferred over a transmission line at high frequencies, a filter is placed at the receiving end of a transmission. The filter provides compensation to the signal being received, so that the distortions caused by the transmission line are removed. Ideally, the filter has a transfer function that substantially offsets the transfer function of the transmission line. As a result, the filtered signal is substantially the same as the signal provided at the input of the transmission line.
In data communications applications, such as IEEE 802.3u compliant networks, it is further desirable for the transfer function of the filter, to compensate for the different distortions provided by different lengths of transmission line. As shown in FIG. 1, different length transmission lines provide different transfer functions affecting signal amplitude and phase shift.
In the case of filters for data communications applications, it is also desirable for the filter to be implemented using complimentary metal oxide semiconductor ("CMOS") technology that is targeted for digital applications. This will enable the filter to be designed for low power operation. Further, the filter could be integrated onto a single wafer die along with other digital circuits required for implementing an IEEE 802.3u Standard network, such as a transceiver, data terminal equipment node, or repeater.
FIG. 3 illustrates a filter 130 that has a transfer function with poles and zeros that are dependent on specific values of resistors and capacitors employed in the filter 130. The filter 130 includes an operational amplifier ("op-amp") 135 having an output (VOUT) which provides the output of the filter 130. A first input (VPOS) of the op-amp 135 is coupled to ground, while a second input (VNEG) is coupled to two different sets of resistors and capacitors.
One set of a resistor and capacitor includes a resistor 131 having a resistance of R1 connected in parallel to a capacitor 132 having a capacitance C3. Resistor 131 and capacitor 132 each have one end connected to an input signal VIN of the filter 130 and another end connected to the second input (VNEG) of the op-amp 135. The other set of a resistor and a capacitor includes a resistor 133 having a resistance R2 coupled in parallel to a capacitor 134 having a capacitance C4. Resistor 133 and capacitor 134 each have one end connected to the second input (VNEG) of the op-amp 135 and another end connected to the output of the op-amp 135.
The transfer function of a filter is the ratio of the filter's output to the filter's input. Transfer functions for filters are typically expressed in terms of their s-domain equivalent, where s is equal to j.omega. and a capacitance is equal to s times the capacitor's capacitance. The transfer function of the filter 130 in FIG. 3 is equal to the following s-domain expression:
VOUT/VIN=(C3/C4)*(s+1/(R1*C3))/(s+1/(R2*C4)) (Equation 1)
The filter in FIG. 3 therefore has the following pole and zero:
Pole=1/(R2*C4) PA1 Zero=1/(R1*C3)
In order for the filter 130 in FIG. 3 to provide adequate compensation for the transmission line distortion that a signal suffers, the value of the filter's transfer function will have to be set to offset the transfer function of the transmission line. In the case of filter 130, this requires selecting precise values for R1, R2, C3, and C4. However, it is very difficult, and sometimes not possible, to form resistors and capacitors in integrated circuits with precise resistance and capacitance values.
Further, there is no mechanism in filter 130 to provide for adjusting the transfer function to account for different lengths of transmission line once the resistors 131, 133 and capacitors 132, 134 are selected. It is also very difficult in CMOS technology targeted for digital applications to provide an op-amp with sufficient high gain bandwidth for operating at frequencies of 125 MHZ.
FIG. 4 illustrates a filter 140 that is not dependent on the specific values of components employed in the filter 140. Instead, the filter's transfer function is dependent upon the ratio of capacitors that are switched into the filter 140. The filter 140 includes a network of capacitors 141, 142, 143, 144 and a set of switches 145, 146, 147, 148. The switches 145-148 may be implemented by using transistors. The switches 145-148 couple and decouple capacitors to the inputs and output of an op-amp 149 to set the filter's transfer function in response to an input signal.
In operation, the filter 140 in FIG. 4 requires the switches 145-148 to be controlled by a sample clock having a frequency much higher than the input signal being provided to the filter 140. This enables the sample clock to effectively sample the incoming signal and set the switches 145-148. When the signal being sampled is in the range of 125 MHZ, the required very high frequency sample clock is difficult, and some times not possible, to provide. Further, the filter 140 still requires the use of an op-amp 149 with sufficient high gain bandwidth for operating at frequencies of 125 MHZ. As described above with reference to FIG. 3, such an op-amp is very difficult to provide in CMOS technology targeted for digital applications.
Accordingly, it is desirable to provide a filter that can be implemented in CMOS technology targeted for digital applications. The filter may also avoid the requirement of precise resistor values to set the pole and zero of the filter's transfer function. It is also desirable for the filter to have a transfer function that can compensate for the distortion suffered by high frequency signals on different length transmission lines.